The present invention is directed to an eddy current braking apparatus and, in particular, to a single-row rotating-type braking apparatus.
Large vehicles such as buses and trucks are equipped with a foot brake as a main braking apparatus. In addition, these large vehicles are equipped with an exhaust brake as an auxiliary braking apparatus (retarder). Furthermore, on a portion of an output shaft or a power transmission shaft such as the rear of the transmission or the midportion of a propeller shaft, these large vehicles are equipped with an eddy current braking apparatus as an auxiliary braking apparatus. An eddy current braking apparatus performs stable deceleration even on a long downhill slope, and it decreases the frequency of use of a foot brake. For this reason, abnormal wear of brake linings and the occurrence of fading can be prevented, and the braking distance can be shortened. Typically, electromagnets and permanent magnets are used as the magnets for generating the magnetic force in these types of eddy current braking apparatuses, with permanent magnets being used more since no electric current is required during braking.
One type of an prior art eddy current braking apparatus used in the prior art is a single-row rotating-type eddy current braking apparatus 1 using permanent magnets as shown in FIG. 30 and disclosed in Japanese Published Unexamined Patent Application Hei 1-298948. The eddy current braking apparatus 1 has a support body 4 made of a non-magnetic material such as an aluminum alloy casting. The body 4 is supported with respect to an output shaft or a power transmission shaft 2 such as the rear of a transmission or a propeller shaft of a large vehicle (collectively referred to below in this specification as a xe2x80x9cpower transmission shaftxe2x80x9d) by bearings 3. The support body 4 has a support ring 5, which serves as a yoke and is made of steel, for example, and is rotatably supported through bearings 6.
A plurality of permanent magnets 7 are disposed on the outer peripheral surface of the support ring 5. The magnets 7 are disposed at constant intervals in the circumferential direction of the support ring 5, with the polarity of adjoining magnets being opposite from each other. A plurality of switching plates 8 made from a ferromagnetic material are magnetically insulated from each other, and are disposed on the support body 4 with a prescribed separation from the outer peripheral surface of the plurality of permanent magnets 7.
A rotor 9 is mounted on the power transmission shaft 2 with the inner peripheral portion of a cylindrical surface 9a thereof opposing the switch plates 8 with a prescribed separation therefrom. The support ring 5 is designed to rotate by only a prescribed angle with respect to the support body 4.
Still referring to FIG. 30, reference number 4a indicates an installation portion for installing the support body 4 on its mounting member, and reference number 9b indicates a cooling fin for radiating heat of the rotor 9.
FIG. 31 shows another type of an eddy current braking apparatus 10 that is a two-row rotating type and which is disclosed in Japanese Published Examined Patent Application Hei 7-118901. The apparatus 10 has a rotor 12 mounted on a power transmission shaft 11. A permanent magnet group 13 is supported by a fixed support ring 14 from the fixed side opposing the rotor 12, and the magnets of group 13 are arranged with a predetermined spacing in the circumferential direction of the fixed support ring 14 with the north poles and the south poles alternating with each other. The apparatus 10 has a moveable support ring 16 which can rotate with respect to the fixed support ring 14. Another permanent magnet group 15 is mounted on the moveable support ring 16. Bearings 18 permit sliding movement between the support ring 16 and the support body 4. A plurality of ferromagnetic switching plates 17 are disposed between the rotor 12 and the permanent magnet groups 13 and 15. The switching plates 17 extend from above the permanent magnets 13 of the fixed support ring 14 to above the moveable support ring 16.
The apparatus of FIGS. 30 and 31 are not without their drawbacks though. Single-row rotating-type braking apparatus such as that shown in FIG. 30 exhibit a drag torque during a non-braking state that is higher than for the two-row rotating-type eddy current braking apparatus 10 shown in FIG. 31. Consequently, single row eddy current braking apparatus 1 have not gained acceptance at the present time.
However, these single row eddy current braking apparatus can be beneficial in that their use can greatly decrease the number of braking components such as permanent magnets 7, support rings 5, and the retaining members that support the permanent magnets 7 and the support rings 5. Thus, the use of these types of eddy current braking apparatus increases durability and reliability, and lowers the cost of manufacturing. Furthermore, the single row eddy current braking apparatus can restrict the magnet rotational angle to about half that of the permanent magnets 15 of the double row eddy current braking apparatus, thereby permitting a decrease in the size of pneumatic cylinders employed for driving the permanent magnets 15 and a decrease in air consumption.
A further advantage of single row eddy current braking apparatus relates to the size of the pneumatic cylinder used to alternate between braking and non-braking states. Switching from a braking state to a non-braking state is typically accomplished simply by rotating the support ring 5 such that two adjoining switching plates 8 are each straddled by a permanent magnet 7. The magnetic force of permanent magnets 7 and the attractive force between the permanent magnets 7 and the opposing switching plates 8 is larger than for two-row eddy current braking apparatus 10 of FIG. 31. Consequently, the stroke of a pneumatic cylinder for driving the support ring 5 of the single row braking apparatus can be reduced by about half. As a result, the length of the pneumatic cylinder can be decreased, thereby reducing costs, installation fits problems, and air consumption. However, although the stroke or magnet rotational angle is reduced with a single-row eddy current braking apparatus, the required force to rotate the magnet is not reduced.
Although the single-row eddy current braking apparatus have advantages over two-row current braking apparatus, another problem of drag torque in the non-braking state exists. Referring to FIGS. 30, 32a, and 32b, the non-braking state in the single-row eddy current braking apparatus is achieved by rotating the support ring 5 from the braking position shown in FIG. 32a so that the permanent magnets 7 straddle adjoining switching plates 8 and overlap half of each one. As shown by the arrows in FIG. 32b, a short circuited magnetic circuit is formed by the support ring 5, adjoining permanent magnets 7, and one of the switching plates 8. Therefore, magnetic flux from the permanent magnets 7 no longer acts on the cylindrical portion 9a, eddy currents do not flow in the cylindrical portion 9a, a so-called non-braking state is assumed, and the braking torque disappears.
To achieve the braking state as shown in FIG. 32a, the support ring 5 is rotated so that the permanent magnets 7 are aligned with the switching plates 8. In this state, a magnetic circuit shown by the arrows is formed by the support ring 5, the adjoining permanent magnets 7, the adjoining switching plates 8, and the cylindrical portion 9a of the rotor 9. As a result, the flux from the permanent magnets 7 acts on the cylindrical portion 9a and generates eddy currents, and a so-called braking state occurs whereby a braking torque is generated.
In the non-braking state shown in FIG. 32b, ideally no braking torque (drag torque), which acts as running resistance, is generated at all. However, in actual practice, as shown by the dashed line in FIG. 32b, a leakage flux occurs where the permanent magnets 7 are not covered by the switching plates 8. This leakage flux acts on the cylindrical portion 9a of the rotor 9 to generate the drag torque.
A solution to the above-described drag torque problem has been proposed in each of Japanese Published Unexamined Patent Application Hei 5-211761, Hei 6-165477, Hei 6-189522, and Hei 6-86534.
Japanese Published Unexamined Patent Application Hei 5-211761, as shown in FIG. 33a, discloses an eddy current braking apparatus having an opening 7a formed in the central portion in the circumferential direction of a permanent magnet 7. The central portion is not covered by a switching plate 8 during a non-braking state. Alternatively, and as shown in FIG. 33b, this same application discloses a permanent magnet 7 with cutouts 7b and 7b formed in both sides in the circumferential direction. By decreasing the area of the portion of the permanent magnet 7 that is not covered by the switching plate, the leakage flux that may be present when portions of the permanent magnets 7 of the apparatus that are not covered by the switching plates 8 in the non-braking state is decreased.
Japanese Published Unexamined Patent Application Hei 6-165477, as shown in FIG. 34, discloses an eddy current braking apparatus whereby magnets 19 having an opposite polarity to that of permanent magnets 7 are disposed in the central portion in the circumferential direction of permanent magnets 7, i.e., the portion not covered by the switching plates 8 during a non-braking state. With this type of a magnet, a short circuited magnetic circuit between permanent magnets 7 and magnets 19 appears during the non-braking state, and the generation of drag torque is suppressed.
Japanese Published Unexamined Patent Application Hei 6-189522, as shown in FIG. 35, discloses an eddy current braking apparatus wherein the magnets 7 have a recess 7c formed in the central portion in the circumferential direction of the magnet 7, which is not covered by a switching plate 8 during a non-braking state. The recess 7c reduces the volume of the portion of the permanent magnet 7, which is not covered by the switching plate 8. Therefore, the leakage flux from the portion that is not covered by a switching plate 8 during a non-braking state is decreased.
Japanese Published Unexamined Patent Application Hei 6-86534, as shown in FIG. 36, discloses an eddy current braking apparatus wherein ferromagnetic pole materials 20 are provided at each end of each permanent magnet 7. The materials 20 project towards the switching plates 8 from the outer peripheral surface of each permanent magnet 7 at both ends thereof in the circumferential direction. The leakage flux from the portion which is not covered by the switching plates 8 in a non-braking state is decreased as a result of the increased spacing between the magnets 7 and the rotor 9a. 
While the aforementioned prior art apparatus may reduce the drag torque in a non-braking state, these apparatus suffer from a lack of braking torque during the braking state.
More specifically, each of the apparatus disclosed in Japanese Published Unexamined Patent Applications Hei 5-211761, Hei 6-165477, and Hei 6-189522, (FIGS. 33-35) has a weakening of the magnetic force in the portion of the permanent magnets 7 not covered by the switching plates 8 in a non-braking state, i.e., in the central portion in the circumferential direction. Therefore, while it is in fact possible to obtain a decrease in leakage flux during a non-braking state, the braking torque during a braking state necessarily decreases as well. Furthermore, it is necessary to form difficult-to-work permanent magnets into complicated shapes, and manufacturing costs increase.
In the eddy current braking apparatus disclosed in Japanese Published Unexamined Patent Application Hei 6-86534 (FIG. 36), the separation between the switching plates 8 and the permanent magnets 7 unavoidably becomes large. Therefore, the distance between the permanent magnets 7 and the cylindrical portion 9a of the rotor 9 also becomes large, and the magnetic flux density, which reaches and acts on the cylindrical portion 9a of the rotor 9 during a braking state decreases. In particular, a gap formed between the switching plates 8 in the central portion in the circumferential direction of the permanent magnets 7 all the more decreases the flux, which is generated in the central portion in a circumferential direction of the permanent magnets 7. For this reason, the braking torque during a braking state decreases.
Besides a loss of braking torque, the differences in the coefficients of thermal expansion for the materials of the apparatus can require an increased amount of high cost magnetic material, thus increasing the overall apparatus cost. Referring again to FIGS. 30-36, this increased amount of magnetic material derives from having to adjust the gap between the switching plates 8 and the permanent magnets 7 and the gap between the support body 4 and the bearings 6 to take into consideration changes in temperature during apparatus operation.
The temperature of an eddy current braking apparatus 1 at the time of starting of a vehicle, assuming an extremely cold region, is xe2x88x9220 to xe2x88x9230xc2x0 C. On the other hand, the temperature of the eddy current braking apparatus 1 during running can rise to 70-90xc2x0 C. due to heating by heat transmitted from the rotor 9. The support body 4 of the eddy current braking apparatus 1 is generally made of an aluminum alloy casting or the like in order to be lightweight, non magnetic, and heat radiating. For this reason, the support body 4 has a high coefficient of thermal expansion, and the support body 4 has much larger changes in dimensions due to temperature variations than the bearings 6 or the support ring 5, each being made of steel.
For this reason, the size xcex51 of a gap between the switching plates 8 and the permanent magnets 7 must be set to a large value taking into consideration the maximum shrinkage of the support body 4. With this arrangement, even at the lowest temperature at the time of starting of a vehicle, even if the support body 4 which is made from an aluminum alloy casting shrinks to its smallest size, the switching plates 8 and the permanent magnets 7 will not interfere with each other. On the other hand, the size xcex52 of a gap between the support body 4 and the bearings 6 must be set to a large value to account for the maximum expansion of the support body 4. With this spacing, even when the support body 4, which is made of an aluminum alloy casting, is heated to a maximum temperature during running and expands by a maximum amount, a suitable gap is maintained for the bearings 6. Furthermore, when the lowest temperature occurs, such as when the vehicle is started, the size of the gap xcex52 increases due to the difference in the coefficient of thermal expansion between the bearings 6 which are made of steel, and the support body 4, which is made of an aluminum alloy casting.
For this reason, in order to set gap size xcex51, it is necessary to take into consideration the maximum value of gap size xcex52. Accordingly, as shown in FIGS. 30-36, in the conventional structure in which the steel support ring 5 is supported on the outer peripheral surface side of the support body 4 made from an aluminum alloy casting, it is necessary to set gap size xcex51 at quite a large value. The size xcex51 of this gap thus causes of an increase in magnetic resistance in the magnetic circuit, which is formed during a braking state, and it decreases the efficiency of the magnetic circuit. In addition, due to the increased gap and reduced efficiency, the amount of the high cost material forming the permanent magnets 7 must be increased, thereby increasing the manufacturing costs of the single-row rotating-type eddy current braking apparatus shown in FIG. 30.
Furthermore, in the conventional structures shown in FIGS. 30-36, shifting between a braking state and a non-braking state is carried out by rotating the support ring 5 with respect to the support body 4 at a high speed. For this reason, in order to prevent the magnets 7 from falling off, it is necessary to strongly secure them to the support ring 5, and a high dimensional accuracy is required of the surfaces in the support body 4 forming the gaps having the sizes xcex51 and xcex52. In addition, the sliding surface of the support body 4 that opposes the bearings 6 must undergo hardening treatment and the like in order to increase its durability. As a result, the manufacturing costs of the eddy current braking apparatus of FIG. 30 are enormously increased.
Another problem associated with single-row rotating type eddy current braking apparatus is the necessity of using the right size actuator for rotation, but at the same time having an actuator that is low cost and small in size. In general, the forces required to cause rotation in single-row rotating-type eddy current braking apparatus require actuators that are large.
In the single-row type braking apparatus, in order to switch between generating and turning off a braking force, it is necessary to rotate the support ring 5. The rotation of the support ring 5 is normally carried out by connecting various types of actuators such as a hydraulic cylinder or a pneumatic cylinder or a drive source such as an electric motor to a yoke link protruding from the side surface of the support ring 5. However, in many cases, a large vehicle such as a truck or a bus is equipped with a compressed air source. For this reason, it is convenient to use a pneumatic cylinder as the drive source for the support ring 5 in a single-row rotating-type eddy current braking apparatus. Furthermore, in order to stop the support ring 5 in at least two stopping positions, a double-acting cylinder is often employed.
FIGS. 37a and 37b show an exemplary double acting cylinder, disclosed in Japanese Published Unexamined Utility Model Application Hei 6-48386, that is intended for use with a two-row rotating-type eddy current braking apparatus. In this cylinder, a stepped piston 22 is arranged within a stepped cylinder 21. In addition, a piston 23 is arranged within an open-ended and smaller diameter cylindrical portion of the piston 22. A rod 24 extending from the piston 23 passes through an opening in the closed end wall of piston 22, through an opening in the large diameter end wall of cylinder 21, and projects to an exterior of the cylinder. By switching the supply of compressed air to both end walls of cylinder 21, a support ring (not shown) connected to the rod 24 can be stopped in a non-braking position, a partial braking position, and a braking position, for a total of three positions. It should be understood that a partial braking position means a condition between a non-braking state and a braking state, i.e., not a complete braking state.
When the support ring 5 is driven by a pneumatic cylinder, as shown in FIG. 32a, it is necessary for the pneumatic cylinder to generate a greater force than the magnetic attraction force generated by the magnetic circuit formed by the support ring 5, adjoining permanent magnets 7, adjoining switching plates 8, and the cylindrical portion 9a of the rotor. FIG. 38 shows the relationship between the stroke s of a pneumatic cylinder and the required force F1 for the pneumatic cylinder to rotate the support ring 5 from a non-braking state to a braking state, to maintain this state, to rotate the support ring 5 from the braking state to the non-braking state, and to maintain that state.
The force F1 that is required in this case varies with the position of the support ring 5. More specifically, the required force F1 reaches a maximum value Fmax at point A between point O (non-braking state) and point E (braking state). Therefore, the pneumatic cylinder for driving the support ring 5 must generate a thrust of at least the required force Fmax at this point A. The force required to release the braking state, i.e., the required force FB at point B is smaller than the force required during braking, i.e., the required force Fmax at point A.
As described above, when a single-row rotating-type eddy current braking apparatus is mounted on a large vehicle, it is conceivable to use a pneumatic cylinder as an actuator for rotatably driving the support ring 5. In this case, the pressure of the compressed air, which is supplied to the pneumatic cylinder is 7-9 kgf/cm2. The force necessary to rotate the support ring 5 by the pneumatic cylinder depends on the size of the magnets (magnetic force) and the rotational speed of the rotor 9 (which is generally in the range of 0-4000 rpm). For this reason, it is necessary to choose a pneumatic cylinder matching the maximum force required within the rotational speed range of the rotor 9. In this manner, the pneumatic cylinder size should match the maximum force required within the normally used rotational speed range of the rotor 9.
However, it is not always possible to conveniently match the pneumatic cylinder to the required force in the confines of the braking apparatus mounting location. An eddy current braking apparatus is frequently installed in a cramped region such as within an engine compartment or beneath a floor. For this reason, it is desired that a single-row rotating-type eddy current braking apparatus have a small size. As one step in decreasing the size of an eddy current braking apparatus, it is desirable to decrease the size of the pneumatic cylinder. For this reason, a pneumatic cylinder having as small a force as possible and as low consumption of air as possible, i.e., one having a small size and a high efficiency is required. However, if such a small pneumatic cylinder is used, there is the danger of it being unable to generate the maximum force needed in the normal rotational speed range of the rotor 9.
With a conventional single-row rotating-type eddy current braking apparatus, if it is attempted to decrease the cost and size of the overall apparatus by decreasing the size of the pneumatic cylinder for rotationally driving the support ring 5, there is the danger that the force of the pneumatic cylinder will be inadequate and the support ring can not be driven with certainty. On the other hand, if a pneumatic cylinder which can generate a prescribed force is used so as to rotationally drive the support ring with certainty, decreases in costs and size cannot be expected. Accordingly, in the known single-row rotating-type eddy current braking apparatuses, it is difficult to achieve both operation of the support ring 5 with certainty and decreases in the costs and the size of the overall apparatus at a high level.
The external force applied to the support ring 5 is determined by adding the reaction force of the braking torque received by the support ring 5 and the magnetic repulsive force (attractive force) generated due to the overlap between the permanent magnets 7 and the switching plates 8. For this reason, the position maintaining force for maintaining the piston 23 stopped at the partial braking position M must be larger than the external force applied to the support ring 5. Here, the position maintaining force is obtained as the difference between the force on the stepped piston 22 and on the force on piston 23, which is fit into the hollow portion thereof when in the position shown in FIG. 37b. 
When the piston 23 is moved from a non-braking state shown by point O in FIG. 38 to the partial braking position M, a piston 23 having a diameter which can generate a thrust exceeding the required force Fmax at point A is necessary. Furthermore, in order to stop and maintain piston 23 at the partial braking position M, a stepped piston 22 having a diameter so as to just stop the thrust of piston 23 is necessary.
For this reason, if an actuator like that shown in FIGS. 37a and 37b having three operating positions is used, in order to increase the stability at the partial braking position M, it is necessary to make the diameter of stepped piston 22 considerably larger than the diameter of piston 23. For this reason, the diameter of the actuator itself becomes large and the amount of air that is consumed becomes extremely excessive. Accordingly, it becomes difficult to mount it on a large vehicle.
On the other hand, moving to and maintaining the partial braking position M from E (braking state) against the resistance of piston 23, a stepped piston 22 having a large diameter is necessary. For this reason, the diameter of the actuator becomes large and the amount of air that is consumed becomes excessive, and it is difficult to mount it on a large vehicle.
The inability to maintain the partial braking position can also affect the responsiveness of movement for the other braking states as well. During operation, the external force that is received by the support ring 5 varies with the rotational speed of the rotor 9. Therefore, when a position maintaining force cannot be adequately obtained at the partial braking position M, in a low speed range or a high speed range that is the prescribed rotational speed range of the rotor 9, the responsiveness of movement to the three positions decreases, and an impediment to the movement itself exists.
The difficulty in moving a single-row rotating-type eddy current braking apparatus versus a two-row apparatus is shown graphically in FIG. 39. More particularly, the required force F2 needed to rotate the support ring 5 and the stroke s of a cylinder for a single-row rotating-type eddy current braking apparatus is compared to a two-row rotating-type eddy current, wherein the same outer diameter and the same braking force are used.
The required force F2 at the peak point A when moving from a non-braking state to a braking state is approximately 10-30 percent higher for a single-row rotating-type (shown by a dashed line) than for a two-row rotating-type (shown by a solid line) For this reason, when the two-row rotating-type eddy current braking apparatus actuator shown by FIGS. 37a and 37b is used for a single-row rotating-type eddy current braking apparatus, it is necessary to increase the diameter of piston 23 and stepped piston 22, and to increase the amount of air for operation. As a result, the actuator size is increased, and it is difficult to mount it on a large vehicle.
In light of the above, it is clear that single-row rotating-type eddy current braking apparatus offer benefits over two-row rotating-type eddy current braking apparatus in terms of fewer components, lower cost, less magnet rotational angle, etc. Another problem is that a single-row type eddy current braking apparatus requires much force to rotate the magnets. Other problems with the single-row types of braking apparatus include drag torque, low braking torque, difficulty in keeping magnets in place, high costs due to increased magnetic material when overcoming material expansion problems, the need to manufacture complex shapes when trying to reduce drag torque, the inability to use a small sized pneumatic cylinder, and the like. Consequently, there is still a need for improved eddy current braking apparatus, particularly single-row rotating-types.
In light of the existing needs for improved eddy current braking apparatus, it is a first object of the present invention is to provide an eddy current braking apparatus in which it is not necessary to increase the thickness of switching plates as the braking torque increases, in which there is as little loss as possible of braking force during a braking state, and in which leakage flux from between adjoining switching plates to the cylindrical portion of a rotor is suppressed during a non-braking state, whereby drag torque can be suppressed.
A further object of the present invention is to provide an eddy current braking apparatus which can satisfy the above objective and which can increase the efficiency of a magnetic circuit by minimizing the size xcex51 of the gap between switching plates and permanent magnets, which can simplify the securing of permanent magnets to a support ring, can omit processing operations that need accuracy and the like.
It is also an object of the present invention to satisfy the above objects and to provide an eddy current braking apparatus which even if it employs a pneumatic cylinder of small size with low consumption of air, can generate a sufficiently large maximum force needed in the rotational speed range of the rotor, and which can achieve a high degree of cost decreases and size decreases for the apparatus as a whole.
Yet another object of the present invention is to satisfy the above objects and to provide an eddy current braking apparatus having a multi-position actuator which does not need an excessive amount of air, which can satisfactorily maintain a position at a partial braking position, and which has a small outer diameter so that it can be used in a single-row rotating-type eddy current braking apparatus.
Other objects and advantages of the present invention will become apparent as a description thereof proceeds.
In satisfaction of the foregoing objects and advantages, the present invention provides, in one embodiment, a single-row rotating-type eddy current braking apparatus equipped with a hollow cylindrical rotor mounted on a power transmission shaft, a ferromagnetic support ring which is disposed inside the rotor, and a plurality of permanent magnets which are installed on the outer peripheral surface of the support ring at constant intervals and which are arranged so that the orientations of the magnetic poles thereof alternate. A plurality of ferromagnetic switching plates are disposed between the plurality of permanent magnets and the rotor at approximately the same installation angles as the installation angles of the plurality of permanent magnets, and are spaced from the plurality of permanent magnets and the rotor. A cylindrical support body, which is made of a non-magnetic material supports the support ring and the plurality of switching plates. The plurality of permanent magnets and the plurality of switching plates are installed so as to be capable of rotating with respect to each other by a prescribed angle with respect to the rotational center of the power transmission shaft. The thickness of the switching plates is T, the width of the switching plates is W, the angle with respect to the center of the power transmission shaft between the same ends of two adjoining switching plates is "THgr", and the angle with respect to the center of the power transmission shaft between two adjoining switching plates is xcex4"THgr". The dimensions and angles of the various components satisfy the relationship 0.75xe2x89xa6(xcex4"THgr"/"THgr")xc3x97(W/T) greater than 1.5 or 2.25, the value 1.5 or 2.25 dependent on the vehicle speed.
When the eddy current braking apparatus is used in a vehicle speed in which the final reduction ratio is at most 3.3, the relationship between the thickness T of the switching plates, the width W of the switching plates, the angle xcex4"THgr", and the angle "THgr" is preferably 0.75xe2x89xa6(xcex4"THgr"/"THgr")xc3x97(W/T) xe2x89xa62.25. For a vehicle speed in which the final reduction ratio is greater than 3.3, the upper limit value is 1.5.
Furthermore, in an eddy current braking apparatus according to this invention, when the width of the permanent magnets is w, the relationship 0.7xe2x89xa6(W/w)xe2x89xa61.5 is satisfied.
In another embodiment, the single-row rotating-type eddy current braking apparatus is equipped with a hollow cylindrical rotor mounted on a power transmission shaft, and a ferromagnetic support ring disposed inside the rotor. The plurality of permanent magnets are installed on the outer peripheral surface of the support ring at constant intervals, and are arranged so that the orientations of the magnetic poles thereof alternate. A plurality of ferromagnetic switching plates are disposed between the plurality of permanent magnets and the rotor at approximately the same installation angles as the installation angles of the plurality of permanent magnets, and are spaced from the plurality of permanent magnets and the rotor. A cylindrical support body, which is made of a non-magnetic material, has an inner cylindrical portion, which is secured to and supports the support ring, and an outer cylindrical portion that is secured to and supports the plurality of switching plates. The inner and outer cylindrical portions form an annular space wherein the magnets and switching plates are disposed to achieve braking or non-braking states. The outer cylindrical portion with the switching plates can rotate by a prescribed angle with respect to the inner cylindrical portion about the rotational center of the power transmission shaft with the outer cylindrical portion being supported by the inner cylindrical portion or the support ring through a bearing.
In another embodiment of an eddy current braking apparatus according to this invention, the support body has an inner cylindrical portion, and an outer cylindrical portion that is secured to and supports the plurality of switching plates. A support ring that supports the permanent magnets is disposed between the inner and outer portions of the support body. The support ring can be rotated by a prescribed angle with respect to the rotational center of the power transmission shaft to effect braking.
As another feature of the invention, a seal member can be employed between the inner and outer cylindrical portions of the support body.
In another aspect of the invention, a bearing is preferably disposed between the support body and the outer surface of the support ring to support the support ring and allow for rotation of the support ring by a prescribed angle about the rotational center of the power transmission shaft.
In yet another aspect of the invention, the support ring is switched between a first state in which the permanent magnets coincide with the switching plates and a second state in which the permanent magnets are positioned between two adjoining switching plates. The support ring is driven in the same rotational direction as the rotor by at least one single-rod double-acting cylinder, and the single-rod double-acting cylinder is installed so that switching from the second state to the first state can be carried out by projection of a piston rod. The plurality of switching plates can also be driven in the opposite rotational direction to achieve the same effect as when the support ring is rotated.
In another form of an eddy current braking apparatus according to the present invention, an overlapping state of the permanent magnets and the switching plates can be switched between a plurality of states by driving the support ring or the plurality of switching plates in a rotational direction by a two-stage actuator assembly. The assembly includes a first actuator having a cylinder with a piston slidably disposed therein, and a piston rod extending from the piston. A second actuator having a cylinder in which the cylinder of the first actuator is slidably disposed so as to function as a piston for the second actuator, the piston rod of the first actuator extending from the cylinder of the second actuator and acting as a piston rod for both actuators. A spring may be disposed between the cylinders of the first actuator and the second actuator and on the piston rod side of a pressure receiving chamber, or the first actuator may be constituted by a plurality of actuators disposed in series.
A spring may be disposed between the cylinders of the first actuator and the second actuator and in the pressure receiving chamber on the piston rod side of at least one of the plurality of actuators.
In another form of an eddy current braking apparatus according to the present invention, an overlapping state of the permanent magnets and the switching plates can be switched between a plurality of states by driving the support ring or the plurality of switching plates in a rotational direction by an actuator having two free pistons slidably fitting around a piston rod having a first end within a cylinder and a second end projecting from the cylinder. A stopper is provided on the midportion of the inner peripheral surface of the cylinder and on the midportion of the piston rod between the free pistons. In this case, a spring may be disposed between the free piston and the stopper provided on the piston rod.
The invention is also an improvement in a method of producing a braking torque in a single row rotating type eddy current braking apparatus wherein a plurality of magnets and switching plates, each radially spaced from a rotational center of a powered shaft and spaced from each other, are aligned with each other to create a magnetic circuit and to impose a braking torque on a rotor surrounding the magnets and switching plates. The improvement comprises rotating the magnets to create the magnetic circuit in a direction of rotation coincident with a rotational direction of the powered shaft, or alternatively, rotating the switching plates in a direction opposite to the rotation of the powered shaft.
In the mode wherein the switching plates rotate, the switching plates and magnets are configured with one or more bearings disposed between one or more faces of a rotatable member supporting the switching plates and one or more opposing faces of a support ring supporting the magnets or a fixed member supporting the support ring. Alternatively and in a mode wherein the magnets rotate, the bearings are disposed between one or more faces of a rotatable member supporting the magnets and one or more opposing faces of a fixed member supporting the switching plates.
A seal can be provided between the switching plate rotatable member and the magnet fixed member, and rotating either the switching plates or the magnets can be accomplished using a single rod double acting cylinder linked to the appropriate rotatable member.
The switching plates or the magnets are rotated in the opposite or same rotational direction of the rotor, respectively, to achieve alignment and produce the braking state. That which is rotated can be then rotated in the opposite direction wherein the magnets and switching plates are misaligned to disrupt the magnetic circuit and create a non-braking state. A partial braking state can be achieved by the rotation the either the switching plates or magnets to a position wherein the magnets and switching plates are partially aligned with respect to each other to form a partial braking state, one that is between the braking state and the non-braking state.
The inventive method also entails controlling the dimensions and angular relationship of the magnets and switching plates to satisfy the relationship noted above.